Stenosis of an artery refers to narrowing of the artery lumen due to plaque formation on the interior wall of the artery. The severity of the stenosis is the fraction of the cross-sectional area of the lumen that is occluded by plaque. Since narrowing is often asymmetrical about the axis of the artery, in order to assess the severity of a stenosis, it is necessary to obtain at least two, and preferably more, images perpendicular to the artery axis from orthogonal perspectives.
In angiography, the arterial lumen is filled with a radio-opaque substance and X-ray images of the arterial tree are obtained from different perspectives. Selection of these perspectives is partly arbitrary and partly a process of trial and error once a stenosis has been observed. However, the overall number of images that can be obtained is limited by time, safety and cost. Usually four to seven projections for the left coronary arterial system and two to four for the right coronary artery are obtained. The operator assesses the severity of the stenosis either on the basis of visual examination of the images or by computer analysis of a single image. Since these projections are in general not perpendicular to the arterial axis, estimation of stenosis severity and its length from these images is usually not accurate.